Patients with Alzheimer's disease (AD) have significant difficulties in their ability to retrieve verbal lexical and semantic knowledge early in the course of the disorder, in addition to the well-described alterations in episodic memory. Delineation of the neural systems that underlie this semantic memory impairment may lead to better understanding of the information processing defect, and the identification of those systems that are particularly vulnerable to early neuropathological change in AD. Because the representation and retrieval of semantic knowledge probably involves a distributed network of neocortical areas, it may be the ideal cognitive system for examining the functional manifestations of cortico- cortical disconnection in AD. The purpose of this project is to investigate the changes in semantic memory function which occur in normal aging and in dementia. Using well understood behavioral tasks which focus on lexical and semantic storage and retrieval, and associated information processing during Positron Emission Tomography (PET) scans, we will compare and contrast the brain regional activation during the performance of cognitive tasks related to lexical/semantic information processing between young (n=10) and old (n=10) normal control subjects, and patients with Probable Alzheimer's disease (n=10), in each of three studies. The AD patients will be matched with the elderly controls in terms of age, education, and sex, add with the young controls in terms of education and sex. The three studies will focus on Visual Confrontation Naming (Study #1), Visual Semantic Processing (Study #2), and Word Reading (Study #3). These studies share the common feature that they will permit study of processes related to semantic memory in AD, and permit the evaluation of hypotheses related to understanding the underlying defect responsible for the semantic memory loss.